Technology Roadmap: Lumber and Value-Added Wood Products
7.3.5 Metal-plate Connected Roof Trusses

Taking over from early trusses that were assembled with nailed plywood gussets, metal plate connectors were invented around 1950. Pre-engineered, metal-plate-connected roof trusses, made from SPF species, rapidly penetrated the Canadian residential market in the 1960s, reaching near saturation by the early 1980s. They offer long, clear spans and allow complex roof designs. Wide use of MSR lumber allows designers to reduce safety factors in their calculations (and therefore costs) without increasing the risk of failure.

Current sales of trusses total about $450 million annually, and the industry involves some 300 fabricators who also distribute or manufacture parallel-chord trusses. With the introduction of microcomputers ten years ago, fabricators have been able to design their own trusses on the basis of programs developed by metal-plate suppliers located in the U.S..

The Wood Truss Council of America estimates that metal-plated wood trusses are now used in over 75 percent of all new residential roofs in U.S. single family construction. This proportion is probably closer to 90 percent in Canada. Wood trusses also dominate the agricultural building market, with some competition from steel trusses. Commercial buildings, where steel is the norm, represent the greatest potential for wood trusses. Given the size of trusses, transportation costs constitute a major obstacle to distant sales, but, over the past few years, Canadian producers have been increasing sales in neighbouring U.S. states.

Innovation in this sector relates mostly to the use of computer tools to design increasingly complex roof structures at minimum cost, to diversify production into floor and wall components, and to optimise production costs.

Existing Technology

Roof trusses are always designed and fabricated for a specific job. Design technicians read plans and use a computer program to design trusses, prepare quotations and produce the cutting list for lumber (predominantly 2 by 3 and 2 by 4). The lumber is cut to the required lengths in multiple head saws. Truss assembly takes place on large tables equipped with jigs, and the connector plates are pressed into the wood by roller presses. In larger perations, the cut-up saws and assembly jigs are computer-controlled.

Quality control is done internally, except in British Columbia and Alberta, where third-party quality control programs have been instituted.

Incremental Technological Innovation

• Define conditions for the use of finger-jointed lumber in trusses.
• Apply scanning/grading techniques to the optimisation of lumber for use in truss fabrication.
• Develop more efficient design tools to help truss fabricators diversify production.
• Investigate design concepts to reduce product bulk and facilitate long-distance shipping.

Breakthrough Technological Innovation

• Develop high performance trusses by combining existing technology with new designs or materials such as fibre-reinforced plastics.